Chert (and Chert-Jasper mix)

Physical Properties

Hardness: 7 (Mohs scale), Color: tan, cream/beige, and reddish-brown, Luster: Dull to waxy, Crystal structure: Trigonal (microcrystalline/cryptocrystalline), Cleavage: None/Conchoidal fracture, Specific gravity: 2.6-2.65

Formation & Geological History

Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) or by chemical precipitation in groundwater replacing original limestone or chalk fabrics. Often found in nodules or bedded deposits within Paleozoic and Mesozoic sedimentary layers.

Uses & Applications

Historically used for making stone tools (arrowheads, scrapers) due to its conchoidal fracture. Today used as a component in construction aggregate, road base, and as decorative gravel or tumbled stones for hobbyists.

Geological Facts

Chert is so hard it can strike sparks from steel, which led to its use in flintlock firearms. The redder varieties are often called jasper, while darker grey varieties are known as flint.

Field Identification & Locations

Identify in the field by its smooth 'shell-like' curved fractures (conchoidal), its ability to scratch glass, and its lack of reaction to acid. Common in riverbeds and limestone outcrops.

Identified on: 4/23/2026

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